1. Field of the Invention
The present invention relates to a group III-V type nitride compound semiconductor device and a method of manufacturing the same, and in particular to a group III-V type nitride compound semiconductor device which has a characteristic in its electrode structure and a method of manufacturing the same.
2. Description of the Background Art
Group III-V type nitride compound semiconductor devices have been conventionally fabricated by using sapphire and spinel as the substrate material. It is also necessary to form a cavity by cleaving in fabricating semiconductor devices, particularly semiconductor lasers. This requires use of a substrate material which has cleavage. However, neither sapphire nor spinel have cleavage. Accordingly, semiconductor lasers are fabricated by using an oxide material, such as LiAlO.sub.2, LiGaO.sub.2, as substrate material.
Such a substrate material as sapphire, spinel, LiAlO.sub.2, LiGaO.sub.2 and the like are, however, insulators. Accordingly, in forming an electrode in a proximity of a substrate (i.e., a lower electrode), various techniques are required as described below.
As one example of technique for forming an electrode in a proximity of a substrate, Japanese Patent Laying-Open No. 8-17803 discloses that a grown film of a group III-V type nitride compound semiconductor is partially removed by dry etching or the like to form an electrode in a proximity of a substrate.
FIG. 34 is a perspective view of a light-emitting diode device as one example of a conventional group III-V type nitride compound semiconductor device, with application of the technique of etching a grown film to form an electrode in a proximity of a substrate.
Referring to FIG. 34, the light-emitting diode device has an n-type contact layer 3, an n-type clad layer 4, a light-emitting layer 5, a p-type clad layer 6 and a p-type contact layer 7 successively deposited on an insulating substrate 1, and also has a p-type electrode 9 formed on the deposited layers. The grown film is partially etched away through to a portion of n-type contact layer 3 and an n-type electrode 68 is formed thereon.
FIG. 35 is a perspective view of a laser diode device as another example of conventional group III-V type nitride compound semiconductor devices, with application of the technique of etching a grown film to form an electrode in a proximity of a substrate.
The laser diode device shown in FIG. 35 has n-type contact layer 3, n-type clad layer 4, an active layer 35 with a quantum well structure, p-type clad layer 6 and p-type contact layer 7 successively deposited on insulating substrate 1, and also has p-type electrode 9 formed on the deposited layers. The grown film is partially etched away through to a portion of n-type contact layer 3, and an n-type electrode 78 is formed thereon.
FIG. 36 schematically shows a flow of current when current flows in a conventional semiconductor device in such a structure.
The semiconductor device shown in FIG. 36 has substrate 1, n-type contact layer 3, n-type clad layer 4, an active layer 5, p-type clad layer 6, p-type contact layer 7, an n-type electrode 88 and p-type electrode 9. As shown in FIG. 36, the flow of current in the conventional semiconductor device thus structured is biased to n-type electrode 88. Accordingly, when the device is particularly applied as a light emitting device, the luminous efficiency is degraded. The semiconductor device shown in FIG. 36 also has two electrodes 88 and 9 at a same side with respect to substrate 1 and thus the chip size is disadvantageously increased.
As another example of the technique of forming an electrode in a proximity of a substrate, Japanese Patent Laying-Open No. 7-221347 discloses that a substrate is partially removed to expose a surface of a gallium nitride based compound semiconductor layer and an electrode closer to the substrate is formed on the exposed, gallium nitride based compound semiconductor layer.
FIG. 37 is a cross section of a light emitting diode device as still another example of conventional group III-V type nitride compound semiconductor devices, with application of the technique of partially removing a substrate to expose a gallium nitride based compound semiconductor layer and forming an electrode in a proximity of the substrate on the exposed semiconductor layer.
The light emitting diode device shown in FIG. 37 has an n-type gallium nitride based compound semiconductor layer 94 and a p-type gallium nitride based compound semiconductor layer 96 successively deposited on sapphire substrate 1, and also has a positive electrode 99 on the deposited layers. Sapphire substrate 1 is partially removed so that the removal reaches n-type gallium nitride based compound semiconductor layer 94, to expose n-type gallium nitride based compound semiconductor layer 94. A negative electrode 98 is formed in contact with the exposed n-type gallium nitride based compound semiconductor layer 94.
A semiconductor device thus structured can solve the disadvantageous increase in chip size as with application of the technique disclosed in Japanese Patent Laying-Open No. 8-17803.
For the technique of partially etching a substrate away to form an electrode, however, the electrode closer to the substrate has a reduced area. Accordingly, current is not uniformly injected into the light emitting layer. While the area of the portion to be etched away can be increased to increase the area of the electrode, the mechanical strength of the substrate material is practically degraded and thus the area of the electrode closer to the substrate cannot be so increased. As a result, the operating voltage of the semiconductor device is disadvantageously increased.